This invention relates to a coating material for coating on the surface of a recording medium used in the inkjet recording method, and more particularly to an inkjet recording apparatus for recording ink on a recording medium coated with such coating material.
The inkjet recording method is a method of recording information where small drops of ink are generated and applied onto a recording medium, such as paper, and all or a portion of the ink is adhered on the recording medium by means of the ink discharge method, such as a sucking system by electrostatic power, a system in which mechanical vibration or displacement is given to the ink with piezoelectric elements, and a system utilizing pressure where the pressure is generated by heating the ink to form bubbles. The inkjet recording method is associated with high speed and high quality multi-color recording.
In the inkjet recording method, in order to avoid an inferior recording quality due to bleeding and/or flowing of adhered ink on the recording medium, a specialized recording medium is used. For example, the recording medium is a specially coated paper on which a layer of inorganic pigment is coated which provides a larger amount of water absorption and color constituent absorption contained in the ink. Alternatively, the recording medium has a special film on which the surface is specially processed for ink-flow resistivity.
In the foregoing prior art recording medium, there exists a problem that without such specialized and limited type of recording medium, a higher quality of recording is not maintained in the inkjet recording method. That is, when users who handle the prior art inkjet recorders misuse the recording medium, they encounter problems in that they fail to achieve high quality recording because of bleeding of the ink and in that fatal contamination occurs within the printing mechanism and the recorder because of flowing of the ink.
Moreover, the high cost of the recording medium because of special processing and/or special materials required poses another problem.
In the inkjet recording method, where the ink is particularized into droplets and discharged from nozzles of a recording head onto the recording medium such as a recording paper, etc., and characters, drawings, etc., are printed on the recording medium, it is required that the particularized ink droplets to be discharged from nozzles of the recording head should be about 30 xcexcm in diameter with homogeneity in size and shape, and that such ink droplets be discharged onto the recording medium constantly and regularly to form an image with a dot array of ink drops. But in order to satisfy the foregoing fundamental functional requisites during practical usage, the inkjet recording apparatus should be free from such problems peculiar to the inkjet recorder as, for example, clogging at orifices of nozzles due to caking of the ink, change in viscosity of the ink due to change in atmospheric temperature and humidity, deterioration of the quality of the ink with age in longer term stock, and evaporation of ink solvent at the ink supply system. Therefore, the selection and design of the ink for the inkjet recorder should be optimized, taking with integrity into account, for example, the recording head, the ink supply system, the recording media, and the head driving condition.
Under the foregoing conditions, two kinds of inks, water base and oil base ink, according to solvent classification, are in use for the inkjet recorders for output devices and for computers and office automation equipment. The main characteristics of water base and oil base ink are as follows: the water base ink has higher stability in discharge due to stronger surface tension and has, therefore, higher selectivity and chemical stability in/with materials for an ink tank and other components for the ink supply systems which have direct contact with the ink. The oil base ink has higher selectivity in and better fixability on the recording media such as a paper and better water resistant property after the recording. However, a vital feature considered in selecting the type of ink is the safety to the human body as a result of its flammable and odor characteristics. For this reason, a water base ink has been widely used.
Since water base ink is discharged through a very fine orifice (approximately 20-50 xcexcm in diameter) which is formed as a hole on the recording head nozzle, its principle components are water soluble dye, such as direct dye and acid dye, different from the printing ink in general use, and water which is the principle solvent. Additional components, such as miscellaneous additive(s), if necessary, may be added. The percentage of water in the water base ink, in practical use, is approximately 80% and the ink discharged through the nozzle of the recording head is fixed to the recording medium through a drying process, usually air drying.
Since the water base ink applied during prior art inkjet recording usually has a high percentage of water, most of the duration time from its discharge through the recording head nozzle until anchoring to the recording medium is for evaporation of the water as the principal component. Atmospheric temperature and humidity affects, of course, the duration of the evaporation and it depends sometimes on the types (and materials) of the recording medium. The length of the duration of evaporation, in turn, reflects directly on the duration time of the image production and quality of image on the recording medium. A plain paper is commonly used as the recording medium. When plain paper is used as the recording medium according to the common usage, a longer duration of time is needed for absorption and drying of the ink.
Since humidity on parts of the plain paper spreads over the surface thereof due to delay in absorption and drying, bleeding, waving, wrinkling, etc., on the paper remarkably increase on the plain paper and blotting is apt to occur due to friction between a feeding roller and the undried ink which is undergoing the drying process. Therefore, an inferior quality image results on the plain paper in prior art inkjet recording.
An object of the present invention is to obtain high quality images during inkjet recording without regard to the type of recording medium used, and to provide an inkjet recorder free from contamination, e.g., inside of the recorder due to flow of the recording ink.
Another object of the present invention relates to an economical coating material for the recording medium for the purpose of inkjet recording with higher quality without regard to the type of recording medium used.
Yet another object of the present invention relates to obtaining high speed and high quality fixing of the ink with stability on the recording medium by shortening the drying time of the ink during inkjet recording by evaporating or drying water which is brought to the recording medium through the discharged ink, as well water which is excessively contained in and/or adhered to the recording medium at the time of inkjet recording.
In order to solve the problems in the conventional art, the inkjet recorder of the present invention provides coating means which coats the coating material upon the recording medium before the recording ink is recorded on the recording medium, the inkjet recorder recording an image on the recording medium by discharging the recording ink according to a recording signal.
Moreover, the coating material according to the present invention, which is coated on the recording medium before the ink is recorded on the recording medium, comprises polyvinyl alcohol which is saponified partially and has a degree of saponification of 60% to 97.5%, or polyvinyl alcohol which is saponified completely having a degree of saponification of 97.5% or more.
The composition formula of polyvinyl alcohol is: 
This product is made chemically through hydrolysis of polyvinyl acetate of which composition formula is: 
A portion or all of the COOR group in the polyvinyl acetate that is replaced with the OH group is known in general as polyvinyl alcohol. Polyvinyl alcohol which is saponified completely refers to a replacement of the COOR group with the OH group by 97.5% or more, and polyvinyl alcohol which is saponified partially refers to a replacement of the COOR group by the OH group by less than 97.5%. The solubility of the polyvinyl alcohol varies in relation to the degree of the replacement of the COOR group with the OH group as described above.
With respect to solubility in water, polyvinyl alcohol which is saponified completely hardly dissolves in cold water at approximately 20xc2x0 C., although it swells itself. However, in water at 80xc2x0 C. or above, polyvinyl alcohol which is saponified completely dissolves. With respect to the ratio of substitution of the OH group for the COOR group, the lower the ratio is the more it dissolves in cold water. That is, the lower the ratio is the higher the solubility ratio of the polyvinyl alcohol is in cold water.
Polyvinyl alcohol hardly dissolves into solvents other than water, but it dissolves into a mixed solvent with methanol or ethanol. Polyvinyl alcohol which is saponified completely dissolves into a mixed solvent of up to 40% with methanol or ethanol. The lower the ratio of substitution of the OH group for the COOR group is, the higher its solubility ratio is in a mixed solvent with methanol or ethanol.
Furthermore, since water is the principal component of the ink for the inkjet recorder which generates tiny ink bubbles and spreads (or sprays) the ink by mechanical vibration using piezo-electric elements or by heating the ink and utilizing pressure to spread (spray) the ink, etc., it is possible to select as the recording medium a coating film that has a certain solubility in water, such as polyvinyl alcohol having the foregoing characteristics.
According to the present invention, the surface of the recording medium is coated with a coating film of a material comprising polyvinyl alcohol which is saponified partially and of which the degree of saponification is 60% to 97.5%, or polyvinyl alcohol which is saponified completely and of which the degree of saponification is 97.5% or above. When the ink is sprayed on the coating film, the ink infiltrates into the coating film and dries quickly without flowing or bleeding on the recording medium.
The characteristics of the coating film changes due to the degree of polymerization of polyvinyl alcohol saponified partially and having a degree of saponification of 60% to 97.5% or of polyvinyl alcohol saponified completely and having a degree of saponification of 97.5% or above. In the present invention, inkjet recording can be performed without flowing or bleeding and without regard to the types of materials used for the recording medium on which the coating film is formed. Preferably, the coating material has the following specifications:
(a) 0.2 to 30% content by weight of polyvinyl alcohol saponified partially or polyvinyl alcohol saponified completely (hereinafter referred to as p.p or p.c, respectively) when the average degree of polymerization of p.p or p.c is 150 to 750;
(b) 0.1 to 20% content by weight of p.p or p.c when the average degree of polymerization of p.p or p.c is 750 to 2000; or
(c) 0.05 to 15% content by weight of p.p or p.c when the average degree of polymerization of p.p or p.c is 2000 to 5000.
Polyvinyl alcohol is very stable and less expensive to use as the coating material. This is due to the fact that polyvinyl alcohol is widely used in synthetic paste or as an added agent when the recording paper is made of pulp. Thus the coating materials according to the present invention are economical and allow for the realization of higher quality inkjet recording without regard to the types of recording media used.
Moreover, the coating means facilitates coating of the coating film on the recording medium before the recording ink is recorded. When the recording ink is recorded on the coating film having the foregoing characteristics, high quality inkjet recording is achieved without regard to the type of recording medium used.
In another aspect, the present invention is directed to an inkjet recording apparatus comprising an inkjet recording head for discharging recording ink according to a recording signal and for recording on a recording medium. The inkjet recording head has at least one first orifice for discharging the recording ink, a first passage in communication with the first orifice for receiving a supply of the recording ink, and a heating element for heating the recording ink supplied to the first passage. A coating head coats a coating material on the recording medium prior to recording by the inkjet recording head. The coating head comprises at least one second orifice for discharging the coating material, a second passage in communication with the second orifice for receiving a supply of the coating material, and a heating element for heating the coating material supplied to the passage.
In the inkjet recording apparatus having the foregoing construction, neither special arrangements for coating the coating film nor a complicated configuration of the apparatus is needed. Furthermore, with the inkjet recording head and the coating head having the same structure and the coating material having the foregoing characteristics, high quality recording can be achieved without regard to the type of recording medium used.
Moreover, when the coating material comprises a coating film having the above-mentioned characteristics, the coefficient of viscosity of the coating film may be higher than that of the recording ink, causing the discharge characteristics of the coating head to vary. In such a case, a stable and high quality recording can be achieved, according to the present invention, by providing a control unit for controlling the heating units so that the heating unit for heating the coating film generates a greater amount of heat energy than that generated by the heating unit for heating the recording ink, or by providing the orifice of the coating head with a greater discharge area than the discharge area of the orifice of the inkjet recording head. By providing the coating head with an orifice having a greater discharge area, a higher volume of the coating film can be discharged onto the recording medium, thereby improving the coating efficiency and the overall quality of inkjet recording.
In another aspect, the present invention pertains to an inkjet recording apparatus comprising a coating head for coating a coating material on a recording medium according to a recording signal, a delay circuit for delaying the recording signal for a predetermined amount of time and outputting a delay signal, and an inkjet recording head for discharging recording ink according to the delayed recording signal and for recording on the recording medium. The coating head comprises at least one orifice for discharging the coating material on the recording medium and a passage in communication with the orifice for receiving a supply of the coating material. The inkjet recording head comprises at least one orifice for discharging the recording ink on the recording medium and a passage in communication with the orifice for receiving a supply of the recording ink.
In the present aspect of the invention, the coating material has good compatibility with recording ink and can be coated on a specified area of the recording medium before recording of the recording ink. Furthermore, the coating material according to the present invention can be applied to provide high quality inkjet recording and without regard to the type of recording medium used.
In another aspect, the present invention further comprises a wind pressure generating device and a wind pressure supply device proximate the recording medium and the coating head. By this construction, positive or negative wind pressure can be supplied on the surface of an inkjet recording area of the recording medium during inkjet recording. Thus the excess water which prevents the recording ink from adhering to the surface of the recording medium can be evaporated in a matter of minutes, and the recording ink can adhere to the recording medium with improved stability and high speed to provide high quality images.